In this guide, we’ll explain how to adjust and index your front and rear derailleurs to fix your shifting, as well as demonstrate how to diagnose and fix common shifting problems with your bike gears.
A silent and smooth-running transmission can make all the difference to your ride and fixing your bike’s gears is something that even the most mechanically inexperienced can handle.
How to diagnose common drivetrain problems
The first thing to do is to find out what’s actually wrong with your gears.
Problem: Gears won’t shift up or down perfectly with one click
- Solution: This is most commonly caused by stretched gear cables and you will need to re-index your gears.
Problem: Gears are well indexed but the chain keeps dropping off of either end of the cassette or chainrings
- Solution: This is most commonly caused by poorly set up derailleur limit screws and these will require adjusting. It can also be caused by a bent derailleur hanger.
Problem: My rear gears are properly indexed, but the chain shifts too far in one direction
- Solution: If the chain won’t run to the bottom of the cassette and shifts over the big cog into the spokes (or vice-versa), it sounds like a bent rear hanger needs straightening or replacing. If you’ve been in a crash or damaged the bike somehow, this is the likely cause. It is also possible, but less likely, that the limit screws are causing this.
Problem: Down-shifting is fine but upshifting is sticky or slow
- Solution: When were your cables last replaced? Dirty cables and housing can cause slow or inaccurate shifting.
Problem: Chain slipping, jumping and generally misbehaving
- Solution: Inspect your cassette, chain and chainrings for wear. Can you see a pointy shark-fin-like profile on your cog teeth or chainrings? We’re sorry to say, but that sounds like you may need to replace your chain, cassette and chainrings.
How to index the gears on your bike
If the gears on your bike are properly indexed, each single click of the shifter will cause a single shift up or down the gears, front or rear. If you find you’re skipping a gear or that your shifting is getting stuck, then there’s a good chance you need to adjust your indexing.
Indexing problems can be caused by cable stretch, which is a normal part of the bedding-in process for new bikes or cables.
Check that a bent or damaged rear derailleur isn’t to blame first. If it’s all good, here’s how to index or re-index your gears.
What you need
- A hex key to fit your derailleur cable retention bolt, usually a size 4mm or 5mm
How to index bike gears
Carefully and slowly move the derailleur manually to make sure it won’t allow the chain to drop off either end of the cassette
Shift your front derailleur into the middle or smallest ring, then wind your rear barrel adjuster in fully clockwise, then out one turn. Release the cable mounting bolt on the back of the derailleur.
- With the chain running in the second smallest cog, pull the shifter cable taut and retighten the retention bolt
Pedal gently and push the derailleur until the chain moves up to the second-smallest cog, then stop pedalling. Pull the shifter cable taut and retighten the retention bolt, taking care not to overtighten it, which can cause the cable to fray.
Pedal gently and the chain should return to the smallest cog, then test the indexing by using the shifter to move through the cassette.
If upshifting is hesitant, gradually increase cable tension by turning the barrel adjuster a quarter turn anticlockwise and trying again. If your bike over-shifts, reduce the cable tension by turning your barrel adjuster clockwise until the chain runs smoothly in the second smallest cog.
If upshifting is responsive, but downshifting isn’t, then there’s a high chance the problem is dirty inner cables or housing, in which case you should try lubricating them or replacing them.
Adjust your cable tension using your barrel adjuster until the chain runs smoothly in the second smallest cog
Shift your front derailleur to the smallest ring and your rear to a cog near the middle of the cassette then slacken the front derailleur cable by winding the barrel adjuster fully clockwise.
The barrel adjust will either be at the shifter, inline on the cable or, if it’s a modern front derailleur, one of the bolts on the derailleur itself.
Release the cable retention bolt, take any slack out of the cable and then re-tighten the cable retention bolt.
Attempt to shift to the middle or outer ring. If shifting is hesitant, wind the barrel adjuster one half turn anticlockwise before pedalling again.
Repeat this process until the chain steps up cleanly onto the bigger ring. If you have a third chainring, try upshifting to it and add another half turn if shifting is still hesitant.
Once you’re happy with your upshifting, drop back down through the rings to check your downshifts.
Setting derailleur limit screws
Limit screws control the range of movement of a derailleur. If they’re not set properly, then your chain may be able to drop from the cassette or chainrings, which can be dangerous – for both you and your bike.
Before getting started, ensure your derailleur hanger hasn’t been bent. If it is, you’ll need to fix or replace it.
What you need
- A small crosshead screwdriver or hex key
How to set derailleur limit screws
- Locate the lower limit screw, which will usually be marked with an ‘L’, and screw it fully clockwise
Push on the derailleur to manually shift up to the highest gear possible – this should take you to the second or third biggest ring on the cassette.
While still manually pressing on the derailleur, gently turn the cranks and wind out the low stop screw until the cassette shifts to the biggest cog and isn’t jumping or clicking.
Wind out the high (H) adjuster and allow the chain to move towards the bottom of the cassette by pedalling gently.
Wind in the high adjuster until the chain returns to the smallest ring/highest gear and runs smoothly without jumping or clicking.
Your rear high and low limits should now be set.
Adjust your jockey-wheel clearance by locating the screw on the top-rear of the derailleur. This adjustment is referred to as the B-tension or ‘dangle-angle’.
With the bike in the highest gear, the upper jockey wheel should run just clear of the cassette. Turning clockwise will increase the gap, anti-clockwise will close it.
Adjust until there is about 2mm of clearance between the cassette and the upper jockey wheel.
Shift the front gears to the smallest ring and the back gears to the largest cog. Fully wind in the front derailleur’s low limit (L) adjustment screw.
You should complete this procedure with a slack gear cable, so you can be sure that cable tension isn’t overriding your low limit setting.
To do this, release the cable retention bolt on the derailleur. Slowly turn the pedals and wind out the screw until the chain returns to the small ring and runs smoothly, without rubbing on the derailleur cage.
Pull the slack from the cable and re-tighten the retention bolt, not so tightly that you fray the cable. Wind in the high limit (H) stop screw. Shift the rear gears to the smallest cog.
Pull on the gear cable manually, not with the shifter. You can use a tyre lever to pull on the cable so that it doesn’t dig into your hand.
Keeping pressure on the cable, wind out the screw a half turn at a time until the chain jumps to the biggest ring. After this, quarter turns will allow you to adjust the derailleur so it doesn’t rub the chain.
Your high and low stops are now set, but you will probably need to make adjustments to your front indexing using the barrel adjuster.
Replacing a bent rear derailleur hanger
Derailleur hangers are sacrificial parts that are specifically designed to bend or break in the event of an impact to prevent more costly damage to your derailleur or frame.
If your rear shifting isn’t working properly and your bike has suffered a knock – in a crash or while loading into a car, for example – then there’s a good chance your hanger is bent.
Don’t wait until the bike’s in the stand to check your hanger alignment. Eyeball it regularly – you’ll be catching the mech on things frequently without realising.
A bent hanger can cause the mech to shift into the spokes, with potentially disastrous consequences. Investing in an alignment tool isn’t for everyone, but it could save you a lot of money in the long term.
Bending a hanger is easily done, which is why we’d recommend always carrying a spare with you on the road or trail, as well as a multi-tool with the right size hex keys.
Any hanger will work with any derailleur, but you might need to do some research to find one that is compatible with your frame.
Don’t be tempted to try to bend your hanger back to shape during a ride because it’s likely to break, leaving you with a long walk home.
Instead, you could try readjusting your rear derailleur limits and indexing, by following the videos above, to compensate for the misalignment. This might not give you access to your whole cassette, but it’ll get you home.
Assuming you have a spare to hand, here’s how to install it.
For the more mechanically inclined, it is possible to straighten a hanger but should only be done with the correct tools and the knowledge of how to use them properly.
What you need
- Appropriately sized hex keys
- A new hanger
How to replace a bent derailleur hanger
From behind the bike, sight the line of the chain as it runs over the cassette and through the upper and lower jockey wheels.
All three should be arranged in a straight line, and if they’re not, you’re going to have to go ahead and replace the hanger.
Bends towards the wheel are the most common, though outward misalignment is possible too.
Remove your rear wheel. Then remove the derailleur from the hanger, allowing it to rest on the chain.
Next, undo any bolts connecting the hanger to the frame, taking care not to lose them.
Reverse this process to fit your new hanger, quickly check your limits and indexing, and you’re ready to ride.